Method for manufacturing sensitive elements having a permanent electric polarization and device for performing the same

ABSTRACT

The permanent electric polarization is imparted to the sensitive element by electronic bombardment within an enclosure wherein prevails a rarefied atmosphere and in which said bombardment is followed with a coating of the so-polarized sensitive element with a film of liquid of very low saturating vapor pressure whose dielectric strength is higher than that of the rarefied air of the enclosure and having advantageously lubricating properties.

This is a division of application Ser. No. 812,860, filed July 5, 1977now U.S. Pat. No. 4,173,659.

This invention relates to a method for manufacturing sensitive elementshaving a permanent electric polarization and to a device for carryingout this method.

The sensitive element is made of a material of the "electret" typeobtained for example by polarizing a strip of a thickness from 8 to 25microns, made of dielectric plastic metarial of such a type aspolypropylene, polytetrafluoroethylene, polyethylene terephtalate, etc.. . . This polarization can be effected for example by bombardment ofthe dielectric strip with an electron beam of sufficient energy level.The strip as well as the electron gun is usually placed within anenclosure in which a high vacuum has been effected.

When a normal pressure is progressively restored in the enclosure afterthe polarization operations, the dielectric strength first decreasesaccording to the Paschen law, passes to a minimum value for a pressureof a few Torrs and then, regularly increases until the pressure attainsthe atmospheric pressure value. The charges accumulated in the electretstrip produce a high electric field which ionizes the air particularlywhen the pressure attains the value corresponding to the minimum valueof the dielectric strength. The ionization of the rarefied atmospheresurrounding the strip tends to depolarize it.

A known method for passing from ultra-vacuum to a normal pressureconsists of producing a very quick pressure change so as to limit theduration of the ionization phenomena and accordingly, the degradation ofthe electric charge accumulated in the treated strips. This known methodsuffers from the drawback that the obtained results are oftenheterogeneous.

Known methods for making in a continuous manner sensitive elements towhich is imparted a permanent electric polarization, comprise also thewinding of the polarized strips on a drum. When the strip is wound dry,the turns cannot shift laterally with respect to one another as a resultof the strong adhesive power of the electrets. This results in a badsetting of the strip, further disturbed by numerous folds.

The method of the invention has for object to avoid the above-mentioneddrawbacks.

It consists first to place the element to be polarized in an enclosurecontaining a rarefied atmosphere and imparting thereto a permanentelectric polarization by electronic bombardment. The element is thencontacted, inside the enclosure, with a liquid of very low saturatingvapor pressure whose dielectric strength is higher than that of therarefied air.

At the moment where a normal pressure is restored in the enclosure, thepolarized element is in contact with an electrically rigid liquid whichprotects it against both ionization phenomena and degradation of theelectric charge. Thus, the passage of the highly charged electrets fromultra-vacuum to the normal pressure may be conducted without noticeabledamage.

Another characteristic of the invention consists in the fact that theliquid exhibits lubricating properties. The presence of lubricatingliquid on the sensitized element makes possible a winding without foldsand a good setting of the turns over the take-up reels. The capacity ofthe latter may thus be used more completely.

Other advantages of the method as well as characteristics of the devicefor performing the same will be made apparent from the followingdescription of non limitative embodiments given with reference to theaccompanying drawings wherein:

FIG. 1 shows a first embodiment of the device;

FIG. 2 shows a second embodiment of the device;

FIG. 3 shows a first variant of the first embodiment;

FIG. 4 shows a second variant of the first embodiment, and

FIG. 5 shows a third variant of the first embodiment.

The device shown in FIG. 1 comprises a tight enclosure 1 communicatingthrough an opening 2 with a vacuum pump of a known type, not shown. Theenclosure comprises a tubular portion 3 in which is placed an electrongun 4. A focusing system 5 provides for the concentration of theelectron beam in a plane where is placed the element to be sensitized. Asystem of deflecting coils 6 controls the deflection of the beamaccording to a predetermined deflection law.

The strip to be sensitized is wound up on a delivering reel 7. It istransferred to a take-up reel 8, driven in rotation by a friction roller9, after passage over two rollers 10 arranged in such a manner that thestrip is displaced perpendicularly to the electron beam and in thefocusing plane thereof.

The strip passes over a driving capstan 11 which determines theunwinding speed and then over a roller 12 maintained immersed in avessel or tank 13 containing a coating fluid, consisting for example ofa special oil for ultra-vacuum operations, whose saturating vaporpressure is very low and whose dielectric strength is smaller that thatof the rarefied air. There is used preferably a silicone oil whose vaporpressure is 10⁻⁷ torr. More generally, it is convenient to make use of aliquid whose vapor pressure is at most equal to 5.10⁻⁷ torr.

A heating system, comprising for example electric resistors 14, isimmersed in the tank 13. The strip is wound up on the take-up reel 8,after immersion in oil. A metallic pressing roller 20 is arranged nearthe point of application of the strip on the take-up reel 8. Byexperience, it has been observed that this pressing roller is veryuseful for improving the setting of the turns over one another.

Before the step of sensitizing the strip, a preliminary step isconducted during which the coating fluid is fed to the tank and a veryhigh vacuum is established in enclosure 1. The fluid is heated so as toremove gas therefrom, at a temperature of about 60° C., for exampleuntil the residual pressure in the enclosure becomes lower than 10⁻⁶Torr. The fluid is then allowed to cool down and the strip-drivingsystem is actuated. If a pressure increase above 10⁻⁶ Torr is observedin the enclosure, the unwinding of the strip is discontinued until a newgas removing cycle has been performed.

When the gas removal is sufficient, i.e. when no pressure increase isobserved after the starting of the unwinding operation, the strip to bepolarized is subjected to the electronic bombardment.

When a sufficient amount of the strip has been sensitized, the inside ofthe enclosure is progressively put in communication with the externalmedium at atmospheric pressure and the take-up reel is disengaged andtaken off from the enclosure. The transition time between rarefiedatmosphere and the external pressure is not critical and does not resultin a substantial depolarization of the sensitized strip.

According to the embodiment of FIG. 2, the take-up reel 8 is immersed atleast partly in a tank containing the lubricating coating fluid.

The variants of the first embodiment illustrated in FIGS. 3 to 5,concern three different systems for coating the sensitized strip 15before its winding up on the take-up reel. According to the firstvariant the tank 13 containing the lubricating fluid is place above thestrip 15. The fluid is conveyed through pipe 16 from the tank 13 to apad 17, for example of felt, in frictional engagement with the strip 15.

According to a second variant, the tank 13 is placed below the strip 15and the friction pad 17 is fed by capillarity through a wick 18.

According to the third variant, the sensitized element 15 is coated withlubricating fluid by contact with a roller 19d. The oil is transferredto the latter by a roller 19a dipped into the fluid of the tank, throughtwo intermediary rollers 19b and 19c with which there are in contact.

What we claim is:
 1. A device for manufacturing a sensitive elementhaving a permanent electric polarization comprising a tight enclosurecontaining a plastic element and intermittently communicating with thepumping system for providing a rarefied atmosphere within the enclosure,means for imparting to the element a permanent electric polarization andmeans for supporting the element within the enclosure, the device alsocomprising means for depositing on the element, after polarization, afilm of lubricating liquid of very low saturated vapor pressure whosedielectric strength is higher than that of rarefied air, contained inthe vessel within the enclosure and the vessel including means forheating the liquid contained therein.
 2. A device according to claim 1,wherein said element is a strip of plastic substrate and the supportmeans comprises a driving system for unwinding the strip in front ofsaid polarization means between a delivering reel and a take-up reel. 3.A device according to claim 1, wherein the means for depositing a thinfilm of liquid is placed between the delivering reel and the take-upreel.
 4. A device for manufacturing a sensitive element having apermanent electric polarization comprising a tight enclosure containinga plastic element wound on a delivering reel and intermittentlycommunicating with a pumping system for providing a rarefied atmospherewithin the enclosure, means for imparting to the element a permanentelectric polarization, a take-up reel and a system for driving theelement from the delivering reel in front of the polarization means andthen to the take-up reel, means for depositing on the polarized elementa thin film of lubricating liquid of very low saturated vapor pressurewhose dielectric strength is higher than that of rarefied air, saidliquid being contained in a vessel that is within the enclosure and thatis provided with means for heating the liquid therein and a roller forpressing the element against the take-up reel, said roller being placedin the vicinity of the point where the element comes in contact with thetake-up reel.
 5. A device according to claim 3, wherein said means fordepositing a thin film of liquid on the element comprise a pad and meansfor feeding said pad with lubricating liquid.
 6. A device according toclaim 5, wherein the means for feeding the pad comprises duct meanscommunicating with said vessel.
 7. A device according to claim 5,wherein the means for feeding the pad comprises a wick.
 8. A deviceaccording to claim 3, wherein said means for depositing a thin film ofliquid on the element comprises an assembly of intercontacting rollers,one of said rollers being dipped in the lubricating liquid of thevessel, another one being in contact with said polarized element.
 9. Adevice for manufacturing a sensitive element having a permanent electricpolarization comprising a tight enclosure that contains a plasticelement wound on a delivering reel and that intermittently communicateswith a pumping system for providing a rarefied atmosphere within theenclosure, means for imparting to the element a permanent electricpolarization, a take-up reel and a system for driving the element fromthe delivering reel in front of the polarization means and then to thetake-up reel, means for depositing on the polarized element a thin filmof lubricating liquid of very low saturated vapor pressure whosedielectric strength is higher than that of rarefied air, the liquidbeing contained in a vessel that is within the enclosure and that isprovided with means for heating the liquid therein, and the take-up reelbeing immersed at least partly in the the lubricating liquid containedin said vessel.